The present invention relates to a gas turbine having a closed-circuit cooling system for one or more nozzle stages and particularly relates to a gas turbine having closed-circuit cooling with localized cooling of nozzle wall portions.
Gas turbine nozzles are often provided with open and/or closed-circuit cooling systems. In an open system, for example, an air-cooled nozzle, compressor discharge air is typically supplied to the nozzle vane and exhausted into the hot gas stream. Local air-film cooling is provided to afford improved cooling in localized areas on the airfoil as necessary and desirable. In closed-circuit nozzle cooling systems, a cooling medium, e.g., steam, typically flows from the outer band through various cavities in the vane, through the inner band and returns via return passages through the cavities in the vane and outer band to a steam outlet. The steam cools the nozzle walls by impingement cooling. An example of a closed circuit steam-cooled nozzle for a gas turbine is disclosed in U.S. Pat. No. 5,743,708, of common assignee herewith, the disclosure of which is incorporated herein by reference. That system also employs an open air cooling system for cooling the trailing edge of the vane.
In a closed circuit cooling system, however, it will be appreciated that toward the end of the closed cooling circuit, effective cooling of various surfaces is diminished. This is principally due to lower impingement pressure ratio and an increased cooling medium temperature along those local surfaces. For example, the walls of the cavities adjacent the cooling medium exhaust to the cooling medium outlet are difficult to effectively cool because they lie at the end of the cooling circuit. The cooling medium has gained significant heat pickup and the pressure ratio has been diminished sufficiently to render the localized impingement cooling less effective than desirable. As a consequence, the external wall temperature of the vane at such location is higher, leading to low-cycle fatigue life at such location. Accordingly, there is a need to effectively cool nozzle walls toward the end of the closed cooling circuit.
In accordance with a preferred embodiment of the present invention, there is provided apparatus and methods for effectively cooling localized surfaces of the nozzle walls located adjacent the end of the closed cooling circuit to improve or increase low-cycle fatigue. To accomplish this, a portion of the cooling medium supplied at the beginning of the closed cooling circuit, i.e., a cooling medium portion at inlet conditions, is diverted to one or more secondary inserts within a cavity of the nozzle vane to cool the localized areas which are otherwise difficult to effectively cool at the end of the closed cooling circuit. Particularly, a secondary insert having impingement openings is located within a nozzle cavity adjacent a localized area, i.e., a hot spot requiring localized cooling and is supplied with cooling medium, e.g., steam which has not yet picked up heat from the vane or lost any pressure. The secondary insert uses the pressure drop across the entire cooling circuit to drive the cooling medium through its impingement openings for impingement-cooling of the localized area. This improves the low-cycle fatigue in the localized area being impingement cooled because cooler steam is applied at a significantly higher pressure ratio resulting in substantial increased cooling than otherwise using essentially spent cooling steam at the end of the closed cooling circuit. It will be appreciated that the main insert in the vane cavity and, as illustrated in the prior above-identified U.S. patent, receives the cooling medium, e.g., steam, from the inner band for flow through the insert for impingement-cooling of the vane walls adjacent the main insert. The secondary insert is disposed adjacent a localized hot spot in lieu of impingement-cooling by the main insert at such localized area to supply cooler steam at a higher pressure ratio and, hence, more effectively cool such localized area.
In accordance with a preferred embodiment hereof, there is provided, in a gas turbine nozzle having inner and outer bands and a vane extending therebetween having at least one cavity between side walls of the vane, an insert within the cavity and extending from the outer band and along and spaced from one of the side walls of the vane terminating within the cavity short of one-half the length of the vane, the insert defining a passage for receiving a cooling medium and having openings through a wall thereof for flowing the cooling medium therethrough to impingement-cool the one side wall of the vane and a passage for exhausting spent impingement cooling medium from the vane cavity.
In accordance with another preferred embodiment hereof, there is provided, in a gas turbine having inner and outer bands and a vane extending therebetween having at least one cavity between side walls of the vane, a first insert within the one cavity for receiving a cooling medium, the insert having lateral walls spaced from the side walls and a plurality of openings therethrough for flowing a cooling medium through the openings to impingement-cool the side walls of the vane, and a second insert within the one cavity and having a lateral wall in spaced opposition to one of the side walls with a plurality of openings therethrough for flowing a cooling medium therethrough to impingement-cool a portion of the one side wall.
In a further preferred embodiment hereof, there is provided, in a gas turbine having inner and outer bands, a vane extending therebetween having at least one cavity between side walls of the vane and a closed circuit cooling system for flowing a cooling medium through the vane to cool the vane, a method of cooling a localized area along the vane wall comprising the steps of flowing a first portion of the cooling medium through a first insert in the one cavity for impingement cooling a first portion of the side walls of the vane; flowing a second portion of the cooling medium through a second insert in the one cavity for cooling the localized area of the vane wall, and supplying the second portion of the cooling medium to the second insert at a lower temperature than the temperature of the first portion of the cooling medium supplied to the first insert.
In a still further preferred embodiment hereof, there is provided, in a gas turbine having inner and outer bands, a vane extending therebetween having at least one cavity between side walls of the vane and a closed circuit cooling system for flowing a cooling medium through the vane to cool the vane, a method of cooling a localized area along the vane wall comprising the steps of flowing a first portion of the cooling medium through a first insert in the one cavity for impingement cooling a first portion of the side walls of the vane; flowing a second portion of the cooling medium through a second insert in the one cavity for cooling the localized area of the vane wall, and including supplying the second portion of the cooling medium to the second insert at a higher pressure than the pressure of the first cooling medium portion supplied to the first insert.